1. Field of the Invention
The present invention relates to a method and an apparatus for determining blood pressure of a living body based on sets of data each of which represents a magnitude of a heartbeat-synchronous pulse and a cuff pressure at the time of detection of the pulse.
2. Discussion of the Prior Art
There are known various methods of automatically measuring blood pressue of a living body or subject as pressure in an inflatable cuff set on a body member of the subject is varied, such as (a) "Korotkoff sounds" method in which Korotkoff sounds are detected and blood pressure is determined based on variation in magnitude of the detected Korotkoff sounds; (b) "oscillometric" method in which pressure oscillation transmitted to the cuff synchronously with heartbeat of the subject, namely, pulse wave is detected and blood pressure is determined based on magnitude variation of the detected pulse wave; and (c) "supersonic" method in which vibrations of the wall of an artery are detected by utilizing Doppler effect of supersonic wave and blood pressure is determined based on magnitude variation of the detected artery-wall vibrations.
In each of the above-indicated blood pressure measuring methods, are plotted points representing magnitudes of Korotkoff sounds, pulses of pulse wave or vibrations of artery wall that are heartbeat-synchronous pulses produced from the subject synchronously with heartbeat of the subject. The points are plotted along an axis indicative of pressure in the inflatable cuff (hereinafter, referred to as "cuff pressure"), and a blood pressure evaluation curve is obtained by connecting the plotted points. Blood pressure is automatically determined based on the thus obtained evaluation curve. For example, cuff pressure at the time the evaluation curve exceeds a predetermined magnitude or level, or at the time a difference in magnitude between each pair of adjacent two points becomes maximum, is determined as maximum or minimum blood pressure.
In the above-indicated conventional methods, if a smooth blood pressure evaluation curve were obtained blood pressure determined based on such curve would be accurate. However, generally it is difficult to obtain such smooth evaluation curve, because points representing the heartbeat-synchronous pulses are plotted with dispersion or variation due to various causes other than variation in cuff pressure. Therefore, the conventional methods suffer from the problem that the accuracy of blood pressure measurement is unsatisfactory.